Scanning device and method of scanning an optical beam over a surface

ABSTRACT

An optical scanning device ( 1 ) to scan an optical beam over a surface (S). The optical scanning device ( 1 ) uses a piezoelectric actuator ( 10 ) acting on a platform ( 2 ) that carries a mirror ( 4 ) to pivot the platform ( 2 ) about a pivot ( 8 ). Voltage is applied to the piezoelectric actuator ( 10 ) to pivot the platform ( 2 ) about the pivot ( 8 ). Changes in the applied voltage result in the angle at which the beam is reflected by the mirror ( 4 ) being altered. In this way, the reflected beam (R) can be scanned to different locations on the surface (S). Providing two such optical scanning devices ( 1   a   , 1   b ) or using two piezoelectric actuators ( 10   aa   , 10   bb ) acting on a single platform ( 2 ) enables two dimensional scanning of the surface (S) by the optical scanning device/s ( 1,1   a   , 1   b ). The optical scanning device ( 1 ) of the present invention may be used in refractive eye surgery laser apparatus.

FIELD OF THE INVENTION

The present invention relates to an optical scanning device, a laserapparatus incorporating such an optical scanning device and a method ofscanning an optical beam over a surface to perform material processingof the surface.

The applications of the present invention include, for example, surgicaland medical applications, such as operations for correcting refractiveerrors of the eye by photorefractive keratectomy (PRK) or laser in-situkeratomileusis (LASIK). The present invention also has industrialapplications for material processing. Material processing examples ofindustrial applications of the present invention includephotolithography in microchip manufacture and glass marking.

BACKGROUND OF THE INVENTION

The specification refers to and describes content of U.S. Pat. Nos.5,520,679, 6,339,278, 6,339,470 and 6,342,751. However, neither thedisclosures in those US patents nor the description contained herein ofcontent of those US patents is to be taken as forming part of the commongeneral knowledge solely by virtue of the inclusion herein of thereference to and description of content of those US patents.Furthermore, this specification describes aspects of prior art opticalscanning systems. However, neither such aspects of prior art opticalscanning systems nor the description contained herein of such aspects ofprior art optical scanning systems is to be taken as forming part of thecommon general knowledge solely by virtue of the inclusion herein ofreference to and description of such aspects of prior art opticalscanning systems.

A wide range of lasers are suitable for the above applications,including: excimer lasers, Nd:YAG, Nd:YLF, Er:YAG, Nd:KGW, CarbonMonoxide, and Carbon Dioxide lasers. The wavelengths produced by theselasers range from deep in the ultra-violet (UV) to long infra-red (IR)wavelengths.

A feature that is often common among the use of these lasers formaterial processing is the need to move the laser beam relative to thematerial surface being processed. When the material is not deliberatelybeing moved and the laser beam is being directed to carry out theprocessing, the movement of the laser beam is often performed bygalvanometer or motor driven mirrors and lenses.

In the field of treating refractive errors by laser ablation, J. T. Lin(U.S. Pat. No. 5,520,679) proposed using galvanometer scanners tocontrol a low energy laser beam into an overlapping pattern of adjacentpulses to produce the desired change in the corneal surface. U.S. Pat.No. 5,520,679 states that this allows a smaller, lower cost laser to beused for this procedure. U.S. Pat. No. 5,520,679 also states otheradvantages, including a reduced need for a homogenous beam and betterflexibility in design of the treatment.

Galvanometer scanning excimer lasers are currently one of the mostcommon means for correcting refractive errors using the LASIK surgicalprocedure.

Although galvanometer scanners have been very successful in scanninglasers for reshaping corneal tissue and a large range of otherapplications, they do have some disadvantages. They have a trade-offbetween the size and weight of the mirror being tilted and the speed bywhich the galvanometer can adjust its position. Sometimes this resultsin mirrors that are not large enough for the optical system or usingmirrors that are too thin to maintain their required flatness during thescanning process. Galvanometer scanners also have limited accuracy whenthe desired scan angle is small (less than 3 degrees).

The galvanometer scanners used in refractive lasers generally work wellat the pulse repetition rates currently used, i.e. 200 Hz or below.However, this assumes that the eye is not moving. Tracking the eye hasnow become an important part of producing good results for refractivesurgery. Between each pulse the position of the eye is measured and thenthe scanner position adjusted to compensate for any eye movements beforethe eye moves again. This means that the scanner must be capable ofmoving much faster than when the laser was operating without an eyetracker. These faster response requirements from the scanner go beyondthe response capabilities of galvanometers. This becomes even more of aproblem when the demands of customised surgery require smaller spotsizes to ablate with higher precision and subsequently much higher pulserepetition rates. Galvanometric scanners would not have adequateresponse for such a laser system.

A problem that sometimes occurs in galvanometric scanners if the eyemoves slightly up and down, getting closer or further away from thelaser, is that each pulse may not hit the eye in the correct position.Because of the scan angle, if the eye is too close to the laser systemthen the pulses over-lap more than intended and the total area exposedto the laser is less than intended. If the eye is too far from the laserthe opposite occurs. In either case, the result of the surgery isdegraded.

An alternative drive mechanism to a galvanometer drive mechanism is apiezoelectric drive.

Piezoelectric drives have the advantages of having potentially infiniteprecision and are capable of generating extremely high forces, so coulddrive a large mirror very fast. However, piezoelectric drives also havea number of significant disadvantages, and although they have been usedto scan laser beams, they have not been generally accepted for this typeof application because of these disadvantages.

The main disadvantage of piezoelectric drive systems is their verylimited range of movement. They are therefore not considered to be apotential means of scanning in applications currently performed bygalvanometer scanners. One method that has been used to amplify therange of piezoelectric scanning is to have the piezoelectric crystalspush or pull on the end of a metal plate. The metal plate bends anddeflects a mirror further than the same piezo would move the mirror ifapplied behind the edge of the mirror. A device based on this techniqueis described by Takeuchi et al in U.S. Pat. No. 6,342,751. However, thistype of technique creates a non-linear beam deflection and loses much ofthe potential accuracy of a piezoelectric drive mechanism, and still hasa much smaller range of scanning than galvanometer scanners. These typesof techniques also suffer from reduced response time, stiffness and havea significantly smaller force/load capability.

The second significant problem with piezoelectric drive systems, oractuators, is that they have significant hysteresis. This is normally inthe order of 10% to 15% of the range of the movement. This hysteresis isanother key reason why piezoelectric driven scanners are currently notused in applications requiring fast complex scan pattens, such as lasersystems for refractive surgery. This hysteresis can be corrected byoperating the piezoelectric system in a closed loop fashion. Thisrequires a sensor to measure the movement of the system and then acontroller that adjusts the voltage to the piezoelectric actuator sothat it moves to the desired position. The problem with this is that itsignificantly reduces the response of the system, and its accuracy isreduced to the accuracy of the sensor. In an application in whichtolerances are critical, such as refractive surgery, the hysteresisinduced error can be so large that the piezoelectric signal and positionsensor signal cannot be compared to check the system is operatingcorrectly. So to achieve a redundant check of scanning performance asecond position sensor would need to be used.

Papademetriou, et al in U.S. Pat. No. 6,339,470 describes means forscanning lasers across optical fibres. This US patent also describes useof a piezoelectric stack to adjust the angle of a mirror. However, thisdescription complains of the lack of range of such a scanning mechanismas special effort is required to scan the laser across the entrance of asingle optical fibre. The main scanning mechanism used in the devicedescribed in this US patent relies on acousto-optic deflection of thelaser beam, where that scan range must cover more than one optic fibre(which is smaller than the range across an eye). Acousto-optic scannersare relatively complex, have high optical losses and are not suitablefor many of the wavelengths used for material processing applications.

The background description in U.S. Pat. No. 6,339,278 (Shinohara, et al)describes conventional inclination optical scanners and listsgalvanometers, stepper motors and other mechanisms as examples but notpiezoelectric mechanisms. However, the invention described in this USpatent does use a piezoelectric device, but it is used as a mechanicaloscillator to drive an ultrasonic motor that deflects the laser beam.

DISCLOSURE OF THE INVENTION

In accordance with a first aspect of the present invention, there isprovided an optical scanning device comprising:

a platform,

a mirror provided on said platform to reflect an optical beam incidenton said mirror,

a pivot about which said platform is able to pivot,

at least first piezoelectric actuator means to act on said platform topivot said platform about said pivot in a first direction,

at least first resilient means to bias said platform about said pivot ina second direction opposed to said first direction,

wherein said first piezoelectric actuator means acts on said platform ata location in proximity to said pivot, to pivot said platform such thatthe angle at which said beam is reflected by said mirror is altered toalter the path of the reflected beam to thereby scan the reflected beamin a first plane over a surface.

In an alternative form, the optical scanning device further comprises:

second piezoelectric actuator means to pivot said platform about saidpivot in a third direction,

second resilient means to bias said platform about said pivot in afourth direction opposed to said third direction, and

wherein said second piezoelectric actuator means acts on said platformat a location in proximity to said pivot, to pivot said platform suchthat the angle at which said beam is reflected by said mirror is alteredto alter the path of the reflected beam to thereby scan the reflectedbeam in a second plane over the surface, such that said reflected beamis scannable over said surface in two dimensions.

In accordance with a second aspect of the present invention there isprovided an optical scanning apparatus comprising:

a first optical scanning device, and

a second optical scanning device,

said first optical scanning device comprising:

-   -   a first platform    -   a first mirror provided on said first platform to reflect an        optical beam incident on said first mirror,    -   a first pivot about which said first platform is able to pivot,    -   first piezoelectric actuator means to act on said first platform        to pivot said first platform about said first pivot in a first        direction, and    -   first resilient means to bias said first platform about said        first pivot in a second direction opposed to said first        direction, and

said second optical scanning device comprising

-   -   a second platform    -   a second mirror provided on said second platform to reflect the        optical beam incident on said second mirror,    -   a second pivot about which said second platform is able to        pivot,    -   second piezoelectric actuator means to act on said second        platform to pivot said second platform about said second pivot        in a third direction, and    -   second resilient means to bias said second platform about said        second pivot in a fourth direction opposed to said third        direction,

wherein said first piezoelectric actuator means acts on said firstplatform at a location in proximity to said first pivot to pivot saidfirst platform such that the angle at which said beam is reflected bysaid first mirror is altered to alter the path of the reflected beam tothereby scan the reflected beam in a first plane, and said secondoptical scanning device is arranged such that said second mirrorreceives said beam reflected by said first mirror and said secondpiezoelectric actuator means acts on said second platform at a locationin proximity to said second pivot to pivot said second platform suchthat the angle at which said beam is reflected by said second mirror isaltered to alter the path of the reflected beam to thereby scan thereflected beam in a second plane, such that said reflected beam isscannable over said surface in two dimensions.

In accordance with a third aspect of the present invention there isprovided a laser apparatus comprising:

a laser to emit an optical beam,

a first optical scanning device, and

a second optical scanning device,

said first optical scanning device comprising

-   -   a first platform    -   a first mirror provided on said first platform to reflect an        optical beam incident on said first mirror,    -   a first pivot about which said first platform is able to pivot,    -   first piezoelectric actuator means to act on said first platform        to pivot said first platform about said first pivot in a first        direction, and    -   first resilient means to bias said first platform about said        first pivot in a second direction opposed to said first        direction, and

said second optical scanning device comprising

-   -   a second platform    -   a second mirror provided on said second platform to reflect the        optical beam incident on said second mirror,    -   a second pivot about which said second platform is able to        pivot,    -   second piezoelectric actuator means to act on said second        platform to pivot said second platform about said second pivot        in a third direction, and    -   second resilient means to bias said second platform about said        second pivot in a fourth direction opposed to said third        direction,

wherein said first piezoelectric actuator means acts on said firstplatform at a location in proximity to said first pivot to pivot saidfirst platform such that the angle at which said beam is reflected bysaid first mirror is altered to alter the path of the reflected beam tothereby scan the reflected beam in a first plane, and said secondoptical scanning device is arranged such that said second mirrorreceives said beam reflected by said first mirror and said secondpiezoelectric actuator means acts on said second platform at a locationin proximity to said second pivot to pivot said second platform suchthat the angle at which said beam is reflected by said second mirror isaltered to alter the path of the reflected beam to thereby scan thereflected beam in a second plane, such that said reflected beam isscannable over said surface in two dimensions to thereby scan thereflected beam over a surface to perform material processing of saidsurface by the reflected beam and the optical path of the reflected beamfrom said second optical scanning device to the said surface issubstantially one metre or more in length.

Preferably, a third mirror is provided to reflect the reflected beamreflected by said mirror of said second optical scanning device prior tosaid reflected beam being reflected to said surface.

More preferably, a fourth mirror is provided to receive the reflectedbeam from said third mirror and said fourth mirror reflects saidreflected beam to said surface.

In accordance with a fourth aspect of the present invention there isprovided a laser apparatus comprising:

a laser to emit an optical beam, and

an optical scanning device comprising

-   -   a platform,    -   a mirror provided on said platform to reflect a said optical        beam incident on said mirror,    -   a pivot about which said platform is able to pivot,    -   first piezoelectric actuator means to act on said platform to        pivot said platform about said pivot in a first direction,    -   first resilient means to bias said platform about said pivot in        a second direction opposed to said first direction,    -   second piezoelectric actuator means to act on said platform to        pivot said platform about said pivot in a third direction,    -   second resilient means to bias said platform about said pivot in        a fourth direction opposed to said third direction,

wherein said first piezoelectric actuator means acts on said platform ata location in proximity to said pivot, to pivot said platform such thatthe angle at which said beam is reflected by said mirror is altered toalter the path of the reflected beam to thereby scan the reflected beamin a first plane and said second piezoelectric actuator means acts onsaid platform at a location in proximity to said pivot, to pivot saidplatform such that angle at which said beam is reflected by said mirroris altered to alter the path of the reflected beam to thereby scan thereflected beam in a second plane, said first plane and said second planebeing substantially mutually orthogonal, to thereby scan the reflectedbeam over a surface to perform material processing of said surface bythe reflected beam and the optical path of the reflected beam from saidoptical scanning device to said surface is substantially one metre ormore in length.

Preferably, a second mirror is provided to reflect the reflected beamreflected by said mirror of said optical scanning device prior to saidreflected beam being reflected to said surface.

More preferably, a third mirror is provided to receive the reflectedbeam from said second mirror and said third mirror reflects said beam tosaid surface.

Preferably, the first plane and the second plane are substantiallymutually orthogonal.

Preferably, said first piezoelectric actuator means acts on saidplatform to push said platform and said first resilient means iscompressively or expandably resilient.

Alternatively, said first piezoelectric actuator means acts on saidplatform to pull said platform and said first resilient means iscompressively or expandibly resilient.

Preferably, said second piezoelectric actuator means acts on saidplatform to push said platform and said second resilient means iscompressively or expandably resilient.

Alternatively, said second piezoelectric actuator means acts on saidplatform to pull said platform and said second resilient means iscompressively or expandably resilient.

The optical beam may be a laser beam.

The laser apparatus may be a refractive eye surgery laser apparatus, inwhich case the surface on which the material processing is performed bythe reflected beam is the eye of a patient on which the refractivesurgery is performed by the reflected beam.

However, the laser apparatus may also be used in other medicalapplications, in which case the surface on which the material processingis performed by the reflected beam is (human or animal) tissue. Anotheruse of the laser apparatus is as an industrial laser.

In accordance with a fifth aspect of the present invention there isprovided a method of scanning an optical beam, in at least a firstplane, over a surface using at least one optical scanning device ashereinbefore described comprising

determining a required location for an optical beam to be incident onsaid surface,

determining whether a positive or negative change to the voltage appliedto a said piezoelectric actuator means is required to pivot saidplatform to a required position corresponding to the said requiredlocation,

comparing the existing position of said platform and the voltage appliedto said piezoelectric actuator means with the required position of saidplatform,

calculating the required voltage to be applied to said piezoelectricactuator means corresponding to the required position of said platform,

applying the said required voltage to said piezoelectric actuator meansto move the platform to said required position such that the opticalbeam is incident on said surface at the said required location.

Preferably, said required position of said platform and thecorresponding required voltage to be applied to said piezoelectricactuator means are recorded for use in determining the voltage to beapplied to said piezoelectric actuator means for the next location atwhich said optical beam is to be incident on said surface

In one preferred embodiment of the method of scanning an optical beam,an optical scanning device in accordance with the alternative form ofthe optical scanning device in accordance with the first aspect of thepresent invention is provided to scan the optical beam in two planesover said surface, wherein the steps of the method as hereinbeforedescribed are carried out on each of the first piezoelectric actuatormeans and the second piezoelectric actuator means to pivot said platformto the required position for the optical beam to be incident on saidsurface at said required location.

In an alternative preferred embodiment of the method of scanning anoptical beam, an optical scanning apparatus in accordance with thesecond aspect of the present invention is provided to scan the opticalbeam in two planes over the surface, wherein the steps of the methodhereinbefore described in accordance with the fifth aspect of thepresent invention are carried out on the first piezoelectric actuatormeans of the first optical scanning device and the second piezoelectricactuator means of the second optical scanning device to pivot therespective platform of said first optical scanning device and saidsecond optical scanning device to the required position, respectively,for the optical beam to be incident on said surface at the requiredlocation.

The method hereinbefore described may be used in applications such asperforming refractive eye surgery on a patient, in which case thesurface over which the optical beam is scanned is the eye of thepatient.

The method may also be used in other medical applications, in which casethe surface over which the optical beam is scanned is (human or animal)tissue.

The method may also be used in material processing applications inindustry.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 a is a plan view of a first embodiment of an optical scanningdevice in accordance with an aspect of the present invention;

FIG. 1 b is a cross section view taken along the line A—A in FIG. 1 a;

FIG. 2 are plots of the relationship between the position of theplatform of the scanning device shown in FIG. 1 versus the correspondingvoltage to be applied to the piezoelectric actuator of the scanningdevice shown in FIG. 1

FIG. 3 is a view of an arrangement of two optical scanning devices, ofthe type shown in FIG. 1, arranged so as to scan an optical beam in twodimensions or planes;

FIG. 4 is a view of a second embodiment of an optical scanning device inaccordance with an aspect of the present invention;

FIG. 5 is a schematic illustration of a refractive surgery laserapparatus in accordance with another aspect of the present invention.

BEST MODE OF CARRYING OUT THE INVENTION

In FIGS. 1 a and 1 b, there is shown an optical scanning device 1 thatis able to reflect an incident optical beam I so that the reflectedoptical beam R can be directed to a surface S such that the reflectedoptical beam R is scanned over the surface S.

The optical scanning device 1 comprises a platform 2, a mirror 4 havinga reflective surface 6, a pivot 8 about which the platform 2 is able topivot, a piezoelectric stack actuator 10 to pivot the platform 2 aboutthe pivot 8 in a first direction, and a resilient spring 12 to bias theplatform 2 about the pivot 8 in a second direction that is opposed tothe first direction. The pivot 8 is a pivot shaft.

The optical scanning device 1 is further provided with a body 13 tosupport the other components of the optical scanning device 1. The body13 comprises an anchor block 13 a and a main block 13 b that areseparated by a spacer 13 c. The main block 13 b supports thepiezoelectric stack actuator 10. A pair of O rings 14 are providedaround the piezoelectric stack actuator 10 at the locations where thepiezoelectric stack actuator 10 extends out of the main block 13 b. Acap 15 of low friction material, such as, for example, Teflon, isprovided at the end of the piezoelectric stack actuator 10 that isadjacent the platform 2. The cap 15 and the underside of the platform 2are provided with recesses which accommodate a ball bearing 16. Thepiezoelectric stack actuator 10 thus acts on the underside of theplatform 2 via the cap 15 and the ball bearing 16. The cap 15 isprovided with a recess 15 a which receives and covers the end 16 a ofthe piezoelectric stack actuator 10. The cap 15 prevents sidewaysmovement of the piezoelectric stack actuator 10 when it acts on theplatform 2. A retainer 17 is provided around the mirror 4 to retain themirror 4 in place.

A push-pull amplifier 18 is provided to drive the piezoelectric stackactuator 10. An adjustment screw 19 and locking nut 19 b are provided toadjust the mid position of the scan range of the optical scanning device1.

Voltage can be applied to the piezoelectric stack actuator 10 to expandthe piezoelectric stack actuator 10. The piezoelectric stack actuator 10acts on the platform 2 at a location in proximity to the pivot 8. Forexample, the piezoelectric stack actuator 10 may act on the platform 2at a location that is spaced substantially 5 to 15 mm from the pivot 8.Expansion of the piezoelectric stack actuator 10 pushes on the platform2 to cause the platform 2 to pivot about the pivot 8 in the directionshown by arrow A in FIG. 1. The spring 12 acts to bias the platform 2about the pivot 8 in a second direction, shown by arrow B in FIG. 1,that is opposed to the first direction (shown by arrow A). Thus, when avoltage is applied to the piezoelectric stack actuator 10, thepiezoelectric stack actuator 10 acts against the spring 12 to pivot theplatform 2 in the direction shown by arrow A. Once the applied voltageis removed from the piezoelectric stack actuator 10, the spring 12returns the platform 2 to its original position. If the applied voltageis reduced, rather than removed completely, the spring 12 biases theplatform 2 in the second direction, shown by arrow B in FIG. 1, by anamount corresponding to the reduction in the applied voltage. If theapplied voltage to the piezoelectric stack actuator 10 is increased, thepiezoelectric stack actuator 10 further expands to cause the platform 2to pivot about pivot 8 by a further amount corresponding to the increasein the applied voltage.

The spring 12 may be arranged, as required, to bias the platform 2 in adirection opposed to the direction in which the platform 2 is pivoted bythe piezoelectric stack actuator 10. The spring 12 may be compressivelyor expandibly resilient, as required to provide the biasing action tothe platform 2 in a direction opposed to the direction in which thepiezoelectric actuator 10 acts on the platform 2.

The optical scanning device 1 is able to scan the reflected optical beamR in a first dimension or plane.

The embodiment of the optical scanning device 1 hereinbefore describedand illustrated in FIGS. 1 a and 1 b is arranged such that thepiezoelectric stack actuator 10 acts on the platform 2 to push againstthe platform 2. However, alternatively the scanning device 1 may bearranged such that the piezoelectric stack actuator 10 acts on theplatform 2 to pull the platform 2. In such an arrangement, the ballbearing 16 would be fixed to the platform 2 and the cap 15, which wouldalso be fixed to the end 16 a of the piezoelectric stack actuator 10. Inthat way, when a voltage is applied to the piezoelectric stack actuator10, it would pull the platform 2 in the direction shown by arrow B inFIG. 1. Correspondingly, the spring 12, in such an arrangement, wouldact to bias the platform 2 about the pivot 8 in the direction shown byarrow A in FIG. 1.

For given scan patterns of an optical beam, such as a laser beam, over asurface S, a relationship exists between the voltage applied to thepiezoelectric stack actuator 10 and the pivot position of the platform2, and mirror 4, (also referred to as the “scanning device position” or“scanner position”) and consequently the target, or incident, locationof the reflected optical beam R on the surface S.

The scan patterns have steps between each location that vary in distanceand direction in a quasi random fashion. The steps in the scan patternsequate to different incident locations of the beam R on the surface Sand correspondingly different scanning device positions. The variationof the steps in the scan pattern in a quasi random fashion is achievedby corresponding quasi random changes in the voltage applied to thepiezoelectric stack actuator 10. This is in contrast to a regular scanpattern which uses systematic, i.e. non random changes, in the appliedvoltage. The reason that a quasi random variation is used is that adifferent shape is being sculpted each time the scanning device 1 isused and there is a need to move the laser beam across the surface beingprocessed so that consecutive laser beam pulses do not overlap andthermal loading is spread across both the surface and time. There mayalso be a need to adjust the scan pattern to compensate for movements ofthe surface being treated that may occur.

For a given voltage applied to the piezoelectric stack actuator 10, theposition of the platform 2 may vary by 10% of the full range of movementdue to hystersis occurring in the piezoelectric stack actuator 10,however, the resultant locations are nevertheless reproducible. Thepositions of the platform 2 corresponding to these locations can also bedetermined and are reproducible. It is thus possible to determine arelationship between the scan pattern signal, i.e. the voltage appliedto the piezoelectric stack actuator 10, and the position of the platform2 such that the voltage applied to the piezoelectric stack actuator 10can be adjusted in advance. In this way, the piezoelectric stackactuator 10 can pivot the platform 2, and thereby the mirror 4, into thecorrect required position so that the beam R is incident on the surfaceS at the required incident location.

The relationships between the applied voltage and scanning deviceposition can be determined experimentally for given scan patterns andplotted to produce curves representing the relationships. An example ofrelationships for scanning device position and voltage applied to thepiezoelectric stack actuator 10 is shown in FIG. 2.

The platform 2 of the scanning device 1 may thus be pivoted to thecorrect position without the need to rely on closed loop feedback.However, the scanning device 1, when used in a closed loop fashion,would also increase the response speed of the scanning device 1 andallow a position sensor to operate as a redundant check of the positionof the scanning device 1.

The scanning device 1 may thus be operated as hereinbelow described.

As stated previously herein, for a given current position of thescanning device 1 and a given current voltage applied to thepiezoelectric stack actuator 10, the scanning device 1 will follow apredictable path for changes in the applied voltage to the piezoelectricstack actuator 10. These predictable paths are represented by the curvesin FIG. 2. The path differs for increases or decreases in the voltageapplied to the piezoelectric stack actuator 10 and also depends on thetype of piezoelectric device used in the piezoelectric stack actuator10. These paths are first determined experimentally for a scanningdevice 1 employing a piezoelectric stack actuator 10 having a particularpiezoelectric device.

When it is required to move the incident location of the beam R on thesurface S to a new required location, it is determined whether the newrequired location requires a positive or negative change to the voltageapplied to the piezoelectric stack actuator 10 to pivot the platform 2about the pivot point 8 to a new required position, corresponding to thenew required location of the beam R, so that the beam R will strike thesurface S at the new required location.

The correct path curve is determined that contains the current scanningdevice position and applied voltage at the current position of thescanning device. This determination can be made from the relationshipbetween the scanning device position and the applied voltage, which hasbeen previously determined, i.e. as shown in the plot of scanning deviceposition versus applied voltage, as shown in FIG. 2.

This is then used to calculate the new applied voltage required for thenew required position of the scanning device. The current scanningdevice position and the required applied voltage are then recorded foruse in determining the voltage to apply to the piezoelectric stackactuator device 10 for the next required position of the scanningdevice.

If the current position of the scanning device in unknown, e.g. when thescanning device 1 is first switched on, then a voltage of zero can beapplied which will produce a fixed position that does not depend on theprevious voltage. Effectively, this resets the scanner position to aninitial condition.

When a change in the voltage applied to the piezoelectric stack actuator10 occurs, i.e. a change in the currently applied voltage to the newapplied voltage, the voltage changes directly from the currently appliedvoltage to the new applied voltage, without the applied voltage beingreturned to zero. However, as an alternative, the voltage may bereturned to zero between each change in the applied voltage from thecurrent applied voltage to the new applied voltage.

As previously hereinbefore described, the scanning device 1 is able toscan the beam R in a single dimension or plane. In FIG. 3 there is showna scanning device 1 a and scanning device 1 b. The scanning devices 1 aand 1 b are of the same type as the scanning device 1 as previouslyhereinbefore described with reference to FIGS. 1 a and 1 b.

The scanning devices 1 a and 1 b are arranged such that the beam Rreflected by the first scanning device 1 a can be scanned in a firstdimension or plane and is incident upon the mirror 4 b of the secondoptical scanning device 1 b. The second optical scanning device 1 b isable to reflect the beam R and scan it in a second dimension or plane.The first dimension or plane is substantially orthogonal to the seconddimension or plane. In this way, the arrangement of the scanning devices1 a and 1 b shown in FIG. 3 can scan the reflected beam in twodimensions or planes that are substantially orthogonal to each other.This enables the reflected beam to be scanned in two dimensions over thesurface S.

The piezoelectric stack actuator 10 b (obscured in FIG. 3) of the secondscanning device 1 b pivots the platform 2 b about the pivot 8 b(obscured in FIG. 3) of the second scanning device 1 b in a direction,or about an axis, that is substantially orthogonal to the direction, orabout the axis, that the piezoelectric stack actuator 10 a pivots theplatform 2 a of the first scanning device 1 a. Similarly, the spring 12b biases the platform 2 b about the pivot 8 b of the second scanningdevice 1 b in a direction, or about an axis, that is substantiallyorthogonal to the direction, or about the axis, that the spring 12 abiases the platform 2 a about the pivot 8 a of the first scanning device1 a.

In the arrangement shown in FIG. 3, the first scanning device 1 areceives an incident beam I from a laser 22 and is able to scan thereflected beam R in the plane of the drawing sheet depicting FIG. 3.This reflected beam R is incident upon the mirror 4 b of the secondscanning device 1 b. The beam R is reflected by the mirror 4 b of thesecond scanning device 1 b and can be scanned by the second scanningdevice 1 b in a plane that is substantially orthogonal to the plane ofthe drawing sheet depicting FIG. 3, i.e. the beam R is reflected by themirror 4 b, in a plane, out of the drawing sheet depicting FIG. 3.

The description of the relationship between the voltage applied to thepiezoelectric stack actuator 10 and the pivot position of the platform 2and the previous description herein of the operation of the scanningdevice 1 with reference to FIGS. 1 a, 1 b and 2 applies to the scanningdevices 1 a and 1 b shown in FIG. 3. The scanning devices 1 a and 1 boperate together to enable the reflected beam R to be scanned in twodimensions or planes.

In FIG. 4 there is shown a second embodiment of an optical scanningdevice 11. The optical scanning device 11 is similar to the opticalscanning device 1, except that the optical scanning device 11 isprovided with a pair of piezoelectric stack actuators 10 aa and 10 bband a pair of resilient springs 12 aa and 12 bb and the pivot 8 aballows the platform 2 to pivot in at least two directions, or about twoaxes, that are substantially mutually orthogonal.

The pivot 8 ab may be provided near a corner of the platform 2. Thepivot 8 ab may be provided as a ball that allows the platform 2 toswivel in any direction.

The piezoelectric stack actuators 10 aa and 10 bb and the springs 12 aaand 12 bb may be provided in proximity to the pivot 8 ab. Thepiezoelectric stack actuators 10 aa and 10 bb may be provided such thatthey act on the platform 2 in proximity to the pivot 8 ab, for example,at a location spaced substantially 5 to 15 mm from the pivot 8 ab.

The scanning device 11 permits the platform 2 to pivot about twosubstantially orthogonal dimensions or axes. Thus, the scanning device11 provides an alternative to using two scanning devices 1 a and 1 b, asshown in FIG. 3, to achieve scanning of the beam R about twosubstantially orthogonal dimensions or axes to enable the reflected beamto be scanned in two dimensions or planes.

Voltage applied to the piezoelectric stack actuator 10 aa causes thepiezoelectric stack actuator 10 aa to expand. This in turn, causes theplatform 2 to pivot about the pivot 8 ab in a first direction, shown byarrow C in FIG. 4, against the action of the spring 12 aa. The spring 12aa acts to bias the platform 2 about the pivot 8 ab in a seconddirection, shown by arrow F in FIG. 4, that is opposed to the directionshown by arrow C. Similarly, voltage applied to the piezoelectric stackactuator 10 bb causes the piezoelectric stack actuator 10 bb to expand.This in turn, causes the platform 2 to pivot about the pivot 8 ab in thedirection shown by arrow D in FIG. 1, against the action of the spring12 bb. The spring 12 bb acts to bias the platform 2 about the pivot 8 abin a direction, shown by arrow G in FIG. 1, that is opposed to thedirection indicated by arrow D. The axes about which the piezoelectricstack actuators 10 aa and 10 bb pivot the platform 2 are mutuallyorthogonal. The piezoelectric stack actuator 10 aa and spring 12 aaenable a reflected beam R to be scanned in a first dimension or planeand the piezoelectric stack actuator 10 bb and spring 12 bb enable thereflected beam R to be scanned in a second dimension or plane. The firstdimension or plane is substantially orthogonal to the second dimensionor plane.

In this way, the reflected beam R can be scanned in two dimensions orplanes over a surface S.

The description of the relationship between the voltage applied to thepiezoelectric stack actuator 10 and the pivot position of the platform 2and the operation of the scanning device 1, previously hereinbeforedescribed with reference to FIGS. 1 and 2, also applies to the operationof the scanning device 11.

In FIG. 5 there is shown a refractive eye surgery laser apparatus 20.

The refractive eye surgery laser apparatus 20 comprises an opticalscanning unit (“OSU”), a laser 22 and first and second mirrors 24 and26, respectively.

The OSU may be either an arrangement of two scanning devices 1 a and 1b, as shown in FIG. 3, or a scanning device 11 as shown in FIG. 4. Inthis way, the OSU is able to reflect an incident beam I so that thereflected beam R can be scanned in two dimensions or planes.

The laser 22 emits the laser beam I which is directed to the mirrors 4aa and 4 bb, or mirror 4, of the OSU. The mirrors 4 aa and 4 bb, ormirror 4, reflects the incident beam I as reflected beam R.

The reflected beam R is reflected by the mirrors 4 aa and 4 bb, ormirror 4, of the OSU to a mirror 24. The mirror 24, in turn, reflectsthe beam R to a second mirror 26. The mirror 26 reflects the beam R anddirects it to the surface to be treated, in this case being the eye E ofa patient P.

The OSU is operated, in the manner hereinbefore described with referenceto the scanning device 1 and FIGS. 1 and 2, to scan the beam R over thesurface of the eye E in the required scan pattern to carry outrefractive surgery on the eye E using the laser beam R.

The pivotal movement of the platforms 2 a and 2 b, or the platform 2, ofthe OSU about the pivots 8 a and 8 b, or the pivot 8 ab, causes the pathof the reflected beam R to change with changes in the OSU position. Thechanges in the path of the reflected beam R are preserved by the mirrors24 and 26. In this way, the path of the beam R that is reflected fromthe surface 26 changes. These changes correspond to the changes of theOSU. In this way, the beam R reflected by the mirror 26 can be scannedover the eye E to the required locations where refractive surgery iscarried out by the beam R.

To achieve the required scan range for the beam R reflected from themirror 26 to scan the eye E of a patient P, the optical path between theOSU and the eye E is arranged such that it is substantially one metre ormore. That is to say, the distance travelled by the beam R from the OSUto the mirror 24, from the mirror 24 to the mirror 26 and from themirror 26 to the eye E is substantially one metre or more. It is to beunderstood that by “substantially one metre or more” it is meant thatdistances of slightly less than one metre, as well as distances of onemetre or more, are suitable. This distance enables the refractive lasersurgery apparatus 20, employing the OSU, to scan the range required toperform surgery on the eye E.

The refractive eye surgery laser apparatus 20, hereinbefore described,provides an advantage in processing a material, such as corneal tissueof the eye, that may move or may not be exactly the same distance fromthe laser system each time that the laser 22 is operated to emit a beamI. In that regard, in the OSU, employing a piezoelectrically drivenactuator 10, the scan angle, i.e. the angle through which the beam Rpasses to scan the eye E from one extremity to the other, is muchsmaller than in prior art systems. Therefore, vertical movements ormisalignments of the eye E during surgery will have a much smalleradverse effect on the surgical result compared to prior artgalvanometric scanning systems.

Whilst the scanning device of the present invention has been exemplifiedby its use in a refractive surgery laser system, it may be used in otherlaser apparatus in which a material is processed by a laser beam.

Modifications and variations such as would be apparent to a skilledaddressee are deemed to be within the scope of the present invention.

Throughout the specification, unless the context requires otherwise, theword “comprise” or variations such as “comprises” or “comprising”, willbe understood to imply the inclusion of a stated integer or group ofintegers but not the exclusion of any other integer or group ofintegers.

The invention claimed is:
 1. An optical scanning device for use in performing refractive eye surgery on a patient, characterised in that the device comprises: a platform, a mirror provided on said platform to reflect an optical beam incident on said mirror, a pivot about which said platform is able to pivot, at least first piezoelectric actuator means to act on said platform to pivot said platform about said pivot in a first direction, at least first resilient means to bias said platform about said pivot in a second direction opposed to said first direction, drive means to apply a quasi random voltage to said first piezoelectric actuator means to drive said first piezoelectric actuator means, wherein said first piezoelectric actuator means acts on said platform at a location in proximity to said pivot, to pivot said platform such that the angle at which said beam is reflected by said mirror is altered to alter the path of the reflected beam to thereby scan the reflected beam in a quasi random fashion in a first plane over a surface.
 2. An optical scanning device according to claim 1, characterised in that it further comprises: second piezoelectric actuator means to act on said platform to pivot said platform about said pivot in a third direction, second resilient means to bias said platform about said pivot in a fourth direction opposed to said third direction, and drive means to apply a quasi random voltage to said second piezoelectric actuator means top drive said second piezoelectric actuator means, wherein said second piezoelectric actuator means acts on said platform at a location in proximity to said pivot, to pivot said platform such that the angle at which said beam is reflected by said mirror is altered to alter the path of the reflected beam to thereby scan the reflected beam in a quasi random fashion in a second plane over said surface, such that said reflected beam is scannable over said surface in two dimensions.
 3. A laser apparatus characterised in that it comprises: a laser to emit an optical beam, and an optical scanning device according to claim 1, wherein the reflected beam is scannable over the surface to perform material processing of said surface by the reflected beam and the optical path of the reflected beam from said optical scanning device to said surface is substantially one metre or more in length.
 4. An optical scanning device according to claim 1, characterised in that a cap is provided over an end of said first piezoelectric actuator means that is located proximate said platform to limit sideways movement of said first piezoelectric actuator means proximate said platform.
 5. An optical scanning device according to claim 1, characterised in that the drive means to drive said first piezoelectric actuator means is a push-pull amplifier.
 6. An optical scanning device according to claim 1, characterised in that said first piezoelectric actuator means acts on said platform to push said platform and said first resilient means is compressively resilient.
 7. An optical scanning device according to claim 1, characterised in that said first piezoelectric actuator means acts on said platform to push said platform and said first resilient means is expandably resilient.
 8. An optical scanning device according to claim 1, characterised in that said first piezoelectric actuator means acts on said platform to pull said platform and said first resilient means is compressively resilient.
 9. An optical scanning device according to claim 1, characterised in that said first piezoelectric actuator means acts on said platform to pull said platform and said first resilient means is expandably resilient.
 10. An optical scanning device according to claim 2, characterised in that a cap is provided over an end of said second piezoelectric actuator means that is located proximate said platform to limit sideways movement of said second piezoelectric actuator means proximate said platform.
 11. An optical scanning device according to claim 2, characterised in that the drive means to drive said second piezoelectric actuator means is a push-pull amplifier.
 12. An optical scanning device according to claim 2, characterised in that said second piezoelectric actuator means acts on said platform to push said platform and said second resilient means is compressively resilient.
 13. An optical scanning device according to claim 2, charactensed in that said second piezoelectric actuator means acts on said platform to push said platform and said second resilient means is expandably resilient.
 14. An optical scanning device according to claim 2, characterised in that said second piezoelectric actuator means acts on said platform to pull said platform and said second resilient means is compressively resilient.
 15. An optical scanning device according to claim 2, characterised in that said second piezoelectric actuator means acts on said platform to pull said platform and said second resilient means is expandably resilient.
 16. An optical scanning device according to claim 1, characterised in that said optical beam is a laser beam.
 17. An optical scanning device according to claim 1, characterised in that said first plane and said second plane are substantially mutually orthogonal.
 18. An optical scanning apparatus for use in performing refractive eye surgery on a patient, characterised in that the apparatus comprises: a first optical scanning device, and a second optical scanning device, said first optical scanning device comprising a first platform a first mirror provided on said first platform to reflect an optical beam incident on said first mirror, a first pivot about which said first platform is able to pivot, first piezoelectnc actuator means to act on said first platform to pivot said first platform about said first pivot in a first direction, and first resilient means to bias said first platform about said first pivot in a second direction opposed to said first direction, and first drive means to apply a quasi random voltage to said first piezoelectric actuator means to drive said first piezoelectric actuator means; and said second optical scanning device comprising a second platform a second mirror provided on said second platform to reflect the optical beam incident on said second mirror, a second pivot about which said second platform is able to pivot, second piezoelectric actuator means to act on said second platform to pivot said second platform about said second pivot in a third direction, and second resilient means to bias said second platform about said second pivot in a fourth direction opposed to said third direction, and second drive means to apply a quasi random voltage to said second piezoelectric actuator means to drive said second piezoelectric actuator means, wherein said first piezoelectric actuator means acts on said first platform at a location in proximity to said first pivot to pivot said first platform such that the angle at which said beam is reflected by said first mirror is altered to alter the path of the reflected beam to thereby scan the reflected beam in a quasi random fashion in a first plane, and said second optical scanning device is arranged such that said second mirror receives said beam reflected by said first mirror and said second piezoelectric actuator means acts on said second platform at a location in proximity to said second pivot to pivot said second platform such that the angle at which said beam is reflected by said second mirror is altered to alter the path of the reflected beam to thereby scan the reflected beam in a quasi random fashion in a second plane, such that said reflected beam 5 is scannable over said surface in two dimensions.
 19. An optical scanning apparatus according to claim 18, characterised in that a cap is provided over an end of said first piezoelectric actuator means that is located proximate said first platform to limit sideways movement of said first piezoelectric actuator means proximate said first platform.
 20. An optical scanning apparatus according to claim 18, characterised in that the first drive means to drive said first piezoelectric actuator means is a first push-pull amplifier.
 21. An optical scanning apparatus according to claim 18, characterised in that said first piezoelectric actuator means acts on said first platform to push said first platform and said first resilient means is compressively resilient.
 22. An optical scanning apparatus according to claim 18, characterised in that said first piezoelectric actuator means acts on said first platform to push said first platform and said first resilient means is expandably resilient.
 23. An optical scanning apparatus according to claim 18, characterised in that said first piezoelectric actuator means acts on said first platform to pull said first platform and said first resilient means is compressively resilient.
 24. An optical scanning apparatus according to claim 18, characterised in that said first piezoelectric actuator means acts on said first platform to pull said first platform and said first resilient means is expandably resilient.
 25. An optical scanning apparatus according to claim 18, characterised in that a cap is provided over an end of said second piezoelectric actuator means that is located proximate said second platform to limit sideways movement of said second piezoelectric actuator means proximate said second platform.
 26. An optical scanning apparatus according to claim 18, characterised in that the second drive means to drive said second piezoelectric actuator means is a second push-pull amplifier.
 27. An optical scanning apparatus according to claim 18, characterised in that said second piezoelectric actuator means acts on said second platform to push said second platform and said second resilient means is compressively resilient.
 28. An optical scanning apparatus according to claim 18, characterised in that said second piezoelectric actuator means acts on said second platform to push said second platform and said second resilient means is expandably resilient.
 29. An optical scanning apparatus according to claim 18, characterised in that said second piezoelectric actuator means acts on said second platform to pull said second platform and said second resilient means is compressively resilient.
 30. An optical scanning apparatus according to claim 18, characterised in that said second piezoelectric actuator means acts on said second platform to pull said second platform and said second resilient means is expandably resilient.
 31. An optical scanning apparatus according to claim 18, characterised in that said optical beam is a laser beam.
 32. An optical scanning apparatus according to claim 18, characterised in that said first plane and said second plane are substantially mutually orthogonal.
 33. A laser apparatus comprising: a laser to emit an optical beam, and an optical scanning apparatus according to claim 18, wherein the reflected beam is scannable over a surface to perform material processing of said surface by the reflected beam and the optical path of the reflected beam from said second optical scanning device to the said surface is substantially one metre or more in length.
 34. A laser apparatus for use in performing refractive eye surgery on a patient, characterised in that the apparatus comprises: a laser to emit an optical beam, and an optical scanning device comprising a platform, a mirror provided on said platform to reflect a said optical beam incident on said mirror, a pivot about which said platform is able to pivot, first piezoelectric actuator means to act on said platform to pivot said platform about said pivot in a first direction, first resilient means to bias said platform about said pivot in a second direction opposed to said first direction, first drive means to apply a quasi random voltage to said first piezoelectric actuator means to drive said first piezoelectric actuator means, second piezoelectric actuator means to act on said platform to pivot said platform about said pivot in a third direction, second resilient means to bias said platform about- said pivot in a fourth direction opposed to said third direction, second drive means to apply a quasi random voltage to said second piezoelectric actuator means to drive said second piezoelectric actuator means, wherein said first piezoelectric actuator means acts on said platform at a location in proximity to said pivot, to pivot said platform such that the angle at which said beam is reflected by said mirror is altered to alter the path of the reflected beam to thereby scan the reflected beam in a quasi random fashion in a first plane and said second piezoelectric actuator means acts on said platform at a location in proximity to said pivot, to pivot said platform such that angle at which said beam is reflected by said mirror is altered to alter the path of the reflected beam to thereby scan the reflected beam in a quasi random fashion in a second plane, such that said reflected beam is scannable over said surface in two dimensions to thereby scan the reflected beam over the surface to perform material processing of said surface by the reflected beam and the optical path of the reflected beam from said optical scanning device to said surface is substantially one metre or more in length.
 35. A laser apparatus according to claim 34, characterised in that it further comprises a second mirror to reflect the reflected beam reflected by said mirror of said optical scanning device prior to said reflected beam being reflected to said surface.
 36. A laser apparatus according to claim 35, characterised in that it further comprises a third mirror to receive the reflected beam from said second mirror and said third mirror is arranged to reflect said beam to said surface.
 37. A laser apparatus according to claim 34, characterised in that said laser apparatus is a refractive eye surgery laser apparatus, and the surface on which the material processing is performed by the reflected beam is the eye of a patient on which the refractive surgery is performed by the reflected beam.
 38. A laser apparatus for use in performing refractive eye surgery on a patient, comprising: a laser to emit an optical beam, a first optical scanning device, and a second optical scanning device, said first optical scanning device comprising a first platform a first mirror provided on said first platform to reflect an optical beam incident on said first mirror, a first pivot about which said first platform is able to pivot, first piezoelectric actuator means to act on said first platform to pivot said first platform about said first pivot in a first direction, and first resilient means to bias said first platform about said first pivot in a second direction opposed to said first direction, and first drive means to apply a quasi random voltage to said first piezoelectric actuator means to drive said first piezoelectric actuator, and said second optical scanning device comprising a second platform a second mirror provided on said second platform to reflect the optical beam incident on said second mirror, a second pivot about which said second platform is able to pivot, second piezoelectric actuator means to act on said second platform to pivot said second platform about said second pivot in a third direction, and second resilient means to bias said second platform about said second pivot in a fourth direction opposed to said third direction, and second drive means to apply a quasi random voltage to said second piezoelectric actuator means to drive said second piezoelectric actuator means, wherein said first piezoelectric actuator means acts on said first platform at a location in proximity to said first pivot to pivot said first platform such that the angle at which said beam is reflected by said first mirror is altered to alter the path of the reflected beam to thereby scan the reflected beam in a quasi random fashion in a first plane, and said second optical scanning device is arranged such that said second mirror receives said beam reflected by said first mirror and said second piezoelectric actuator means acts on said second platform at a location in proximity to said second pivot to pivot said second platform such that the angle at which said beam is reflected by said second mirror is altered to alter the path of the reflected beam to thereby scan the reflected beam in a quasi random fashion in a second plane, such that said reflected beam is scannable over said surface in two dimensions to thereby scan the reflected beam over a surface to perform material processing of said surface by the reflected beam and the optical path of the reflected beam from said second optical scanning device to the said surface is substantially one metre or more in length.
 39. A laser apparatus according to claim 38, characterised in that a third mirror is provided to reflect the reflected beam reflected by said second mirror prior to said reflected beam being reflected to said surface.
 40. A laser apparatus according to claim 39, characterised in that a fourth mirror is provided to receive the reflected beam from said third mirror and said fourth mirror is arranged to reflect said reflected beam to said surface.
 41. A method of scanning an optical beam, in at least a first plane, over a surface using an optical scanning device according to claim 1, characterised in that it comprises determining a required location for the optical beam to be incident on said surface, determining whether a positive or negative change to the voltage applied to a said piezoelectric actuator means is required to pivot said platform to a required position corresponding to the said required location, comparing the existing position of said platform and the voltage applied to said piezoelectric actuator means with the required position of said platform, calculating the required voltage to be applied to said piezoelectric actuator means corresponding to the required position of said platform, applying the said required voltage to said piezoelectric actuator means to move said platform to said required position such that the optical beam is incident on said surface at the said required location.
 42. A method according to claim 41, characterised in that said required position of said platform and the corresponding required voltage to be applied to said piezoelectric actuator means are recorded for use in determining the voltage to be applied to said piezoelectric actuator means for the next location at which said optical beam is to be incident on said surface.
 43. A method according to claim 41, characterised in that the steps of the method as cariled out on each of the first piezoelectric actuator means and the second piezoelectric actuator means to pivot said platform to the required position for the optical beam to be incident on said surface at said required location, to thereby scan the optical beam in two planes, such that the optical beam is scannable in two dimensions over said surface.
 44. A method of scanning an optical beam, in two planes, over a surface using an optical scanning apparatus according to claim 41, characterised in that it comprises determining a required location for the optical beam to be incident on said surface, determining whether a positive or negative change to the voltage applied to each of said first piezoelectric actuator means and said second piezoelectric is required to pivot each said platform to a required position corresponding to the said required location, comparing the existing position of each said platform and the voltage applied to said first piezoelectric actuator means and said second piezoelectric means, respectively, with the required position of each said platform, calculating the required voltage to be applied to said first piezoelectric actuator means and said second piezoelectric means, respectively, corresponding to the required position of each respective said platform, applying the said required voltage to said first piezoelectric actuator means and said second piezoelectric actuator means, respectively, to move each said platform to the respective said required position such that the optical beam is incident at the said required location.
 45. A method according to claim 44, characterised in that said required position of each said platform and the corresponding required voltage to be applied to said first piezoelectric actuator means and said second piezoelectric actuator means, respectively, are recorded for use in determining the voltage to be applied to said first piezoelectric actuator means and said second piezoelectric actuator means, respectively, for the next location at which said optical beam is to be incident on said surface. 